A typical wave gear device comprises an annular rigid internal gear, an annular flexible external gear coaxially disposed inside the rigid internal gear, and an elliptically contoured wave generator fitted inside the flexible external gear. The flexible external gear is elliptically flexed by the wave generator, and meshes with the rigid internal gear at the two ends of the elliptical shape in the direction of the major axis. The two gears have a tooth number difference of 2n teeth (where n is a positive integer). Therefore, when the wave generator is rotated by a motor or another rotational drive source, the meshing location of the two gears moves in the circumferential direction, and a relative rotation corresponding to the tooth number difference is generated in the two gears. The rigid internal gear is usually fixed in place, and considerably reduced rotations are output from the flexible external gear.
A wave generator comprises a rotating input shaft connected to a motor or another output shaft, an elliptically contoured rigid plug coaxially mounted on the rotating input shaft, and a wave bearing mounted on the external peripheral surface of the rigid plug. The wave bearing comprises a thin flexible outer ring and inner ring, the inner ring is mounted on the elliptical external peripheral surface of the rigid plug so as to rotate in an integral fashion, and the outer ring is pressed by the rigid plug to the internal peripheral surface of the flexible external gear.
The flexible external gear rotates in a state of being elliptically flexed by the wave generator. Therefore, the frictional resistance exerted on the wave bearing during rotation increases in comparison with the case of perfectly circular rotation. In view of the above, wave bearings are usually ball bearings whose rolling friction resistance in less than that of bearings provided with rolling elements having a roller shape or another shape.
Here, problems such as the following occur when a wave generator provided with ball bearings is rotated at high speed.
(1) The rolling fatigue life of the balls is reduced because of an increase in the number of rolls performed by the ball-bearing balls per unit of time.
(2) Agitation resistance produced by a lubricant on the ball bearings increases, and the heat output increases in association with the agitation. As a result, the sliding portion of the ball bearings is insufficiently lubricated, and the life of the ball bearings is reduced.
(3) The centrifugal force exerted on the balls increases because the orbital speed of the ball-bearing balls increases. The centrifugal force exerted on the balls is particularly increased in large-sized ball-bearing balls. As a result, the rolling fatigue life of the balls is reduced because a considerable load is applied to the outer ring in particular. For this reason, the maximum rotational speed is kept low in large wave gear devices in particular.
In this manner, the life of the wave bearing and other factors impose limits in a wave gear device, making the wave gear device unsuitable for use as a speed reduction device for a gas turbine or other high-speed rotational device. Also, the wave gear device can be used as a speed-increasing device, but the limit of the maximum rotational speed is low and application as a speed-increasing device is therefore limited.